The use of hollow, microporous fibers for the aeration of waste water containing organic pollutants was proposed many years ago, see for example U.S. Pat. No. 4,181,604, dated Jan. 1, 1980, H. Onishi et al.
More recently, it has been proposed to transfer gas to a liquid in a bubbleless manner using hollow, microporous fibers, see for example U.S. Pat. No. 5,034,164, dated Jul. 23, 1991, M. J. Semmens. The bubbleless transfer of gas into the liquid is highly efficient and reduces the loss or waste of gas significantly. Semmens (column 5, lines 27 to 48) teaches the use of a thin, smooth, chemically resistant, non-porous, gas permeable polymer coating on the exterior surface of a major portion of each fiber to inhibit the accumulation of debris and microorganism which tend to clog the surface through which the gas diffuses under high pressures of 20 to 60 psi on the interior of the fibers, while achieving higher gas transfer rates and preventing the loss of gas in bubbles. Semmens further states that if the fibers are uncoated, the pressure differential, that is, the pressure of the gas in excess of that of the liquid, has to be below 2 psi. To avoid bubbles. However, Semmens (column 4, lines 39 to 42) states that generally speaking a gas pressure of at least 45 psi above the water will be used. Clearly, at low gas pressures where no bubbles were formed, the transfer was not considered adequate, and sufficient gas pressure was thought necessary to transfer trapped liquid out of the file membrane (see column 4, lines 34 to 36). While the device of Semmens is useful, the gas permeable polymer coating necessitates the use of elevated gas pressures, while the relatively low liquid pressures will ultimately limit the achievable dissolved gas concentration.
It has also been proposed in U.S. Pat. No. 4,950,431, dated Aug. 21, 1990, A. J. Rudick et al, to provide an apparatus, for making and dispensing carbonated water, in which CO.sub.2, pressurized to 31 psi, from hollow semi-permeable membrane fibers is mixed with chilled municipal water in a carbonator housing. It is stated that as long as the water pressure is equal to or greater than the CO.sub.2 pressure inside the hollow fibers, CO.sub.2 will be absorbed directly into the water without the formation of bubbles (column 4, lines 13 to 31). The CO.sub.2 is provided by an input line having a spring biased spool valve which maintains the interior of the carbonator housing pressurized to the level of the CO.sub.2, i.e., 31 psi, and provides the driving force for dispensing the carbonated water (column 4, lines 2 to 8). Further, when the incoming water pressure is greater than 31 psi to the carbonator housing, the carbonator functions as a simple in-line continuous carbonator during a dispenser operation.
Rudick et al is concerned with producing and dispensing carbonated water which will effervesce at atmospheric pressure. Thus, while CO.sub.2 may be aborbed directly into the water without formation of bubbles, it is necessary for the absorbed portions of CO.sub.2 to be of sufficient size to readily coalesce and effervesce, in the manner of a carbonated beverage, when vented to atmospheric pressure by being dispensed by the Rudick et al apparatus. For this to occur, the carbonated water has to be delivered to the drinking cup in a turbulent state.
While the processes of Semmens and Rudick et al are useful, there is a need to not only further enhance the way that gas is transferred to the liquid, but also to increase the amount of gas available in the liquid by increasing the dwell or residence time during which microscopic portions of the gas remain discrete in the liquid before coalescing and exiting from the liquid in the form of bubbles.